ABSTRACT. The contrasting impacts of externally supplied (runoff) and internally generated (nutrientstimulated phytoplankton blooms) organic matter on oxygen (02) depletion were examined and evaluated in the eutrophic, salinity-stratified Neuse River Estuary, North Carolina, USA. This nitrogen (N)-limited estuary is experiencing increasing anthropogenic N loading from expanding urban, agricultural and industrial development in its watershed. Resultant algal blooms, which provided organic matter loads capable of causing extensive low O2 (hypoxic) and depleted O2 (anoxic) conditions, have induced widespread mortality of resident fin-and shellfish. Phytoplankton blooms followed periods of elevated N loading, except during extremely high runoff periods (e.g. hurricanes), when high rates of flushing and reduced water residence times did not allow sufficient time for bloom development. During these periods, hypoxia and anoxia were dominated by watershed-derived organic matter loading. Externally vs internally generated organic matter loading scenarios were examined in sequential years (1994 to 1996) to compare the differential impacts of an average discharge year ( l 0 yr mean hydrological conditions) (1994), N-stimulated summer algal blooms [1995), and a major hurricane (Fran; September 1996). The responses of primary production, hypoxia, and anoxia to these hydrologically contrasting years and resultant organic matter loadings help distinguish watershed from internal forcing of 0, dynamics and fish kills.

Bacterial abundances, biomass, and production were measured over a 3-yr period at stations along a 15%km reach of the tidal, freshwater Hudson River. Bacterial abundances ranged from 1 to 10 x 1 O6 cells ml-' with maximal values in summer. Abundance and production averaged over all stations for the ice-free season (April through December) were 4.9 and 9.1 x lo9 cells liter-l dm', respectively, and both were significantly correlated with temperature. Neither bacterial abundance nor production showed significant spatial variability over the study reach. In contrast to the results from many autotrophic ecosystems, annual average bacterial abundances from different stations were not significantly correlated with algal standing stocks, and bacterial production was only weakly related to rates of primary production.Absolute rates of bacterial C production were greater than phytoplankton primary production, indicating that much of the bacterial secondary production in this portion of the river must be supported by nonphytoplanktonic organic C.

We examined the effects of anthropogenic and climatic perturbations on nutrient-phytoplankton interactions and eutrophication in the waters of the largest estuarine systems in the U.S.A., the Chesapeake Bay (CB), Maryland/ Virginia, and the Neuse River Estuary/Pamlico Sound (NRE/PS) system, North Carolina. Both systems have experienced large post-World War II increases in nitrogen (N) and phosphorus (P) loading, and nutrient reductions have been initiated to alleviate symptoms of eutrophication. However, ecosystem-level effects of these nutrient reductions are strongly affected by hydrologic variability, including severe droughts and a recent increase in Atlantic hurricane activity. Phytoplankton community responses to these hydrologic perturbations, including storm surges and floods, were examined and when possible, compared for these systems. In both systems, the resulting variability in water residence time strongly influenced seasonal and longer-term patterns of phytoplankton biomass and community composition. Fast-growing diatoms were favored during years of high discharge and short residence time in CB, whereas this effect was not observed during high discharge conditions in the longer residence time NRE/ PS. In the NRE/PS, all phytoplankton groups except summer cyanobacterial populations showed decreased abundance during elevated flow years when compared to low flow years. Although hurricanes affected the CB less frequently than the NRE/PS, they nonetheless influenced floral composition in both systems. Seasonally, hydrologic perturbations, including droughts, floods, and storm-related deep mixing events, overwhelmed nutrient controls on floral composition. This underscores the difficulty in predicting seasonal and longer-term phytoplankton production and compositional responses to nutrient input reductions aimed at controlling eutrophication of large estuarine ecosystems.

An analysis of phytoplankton primary production in the tidal freshwater portion of the Hudson River estuary suggests that net primary production is strongly limited by light and mixing regime. In this turbid, well-mixed system, cells spend from 18 to 22 h d-l below the 1% light level. Autotrophic dark respiration, conservatively estimated at 5% of pb,,,, is of sufficient magnitude to make positive algal growth impossible over much of the river and much of the year. It is particularly difficult to explain the observed increase in algal biomass during blooms in spring and summer. We hypothesize that such blooms can occur only in a small fraction of the river where depth is 54 m.

Three sequential hurricanes, Dennis, Floyd, and Irene, affected coastal North Carolina in September and October 1999. These hurricanes inundated the region with up to 1 m of rainfall, causing 50-to 500-year flooding in the watershed of the Pamlico Sound, the largest lagoonal estuary in the United States and a key West Atlantic fisheries nursery. We investigated the ecosystem-level impacts on and responses of the Sound to the floodwater discharge. Floodwaters displaced three-fourths of the volume of the Sound, depressed salinity by a similar amount, and delivered at least half of the typical annual nitrogen load to this nitrogen-sensitive ecosystem. Organic carbon concentrations in floodwaters entering Pamlico Sound via a major tributary (the Neuse River Estuary) were at least 2-fold higher than concentrations under prefloodwater conditions. A cascading set of physical, chemical, and ecological impacts followed, including strong vertical stratification, bottom water hypoxia, a sustained increase in algal biomass, displacement of many marine organisms, and a rise in fish disease. Because of the Sound's long residence time (Ϸ1 year), we hypothesize that the effects of the short-term nutrient enrichment could prove to be multiannual. A predicted increase in the frequency of hurricane activity over the next few decades may cause longer-term biogeochemical and trophic changes in this and other estuarine and coastal habitats.

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